1. Field of the Invention:
This invention relates to a fan coupling device which controls the rotation of a fan for cooling a car engine in accordance with the external/ambient temperature of the device such that the engine is cooled by the controlled flow of cooling air of the fan.
2. Description of the Prior Art:
A conventional fan coupling device of the above type comprises a closed housing whose inside is divided into two chambers: a torque transfer chamber and an oil basin chamber. A so-called pumping mechanism is formed by a dam member which is attached to the inner circumferential surface of the torque transfer chamber where oil is accumulated during rotating and a circulatory passage which leads from around the dam member inside the torque transfer chamber to the oil basin chamber. A flow adjusting hole leading from the oil basin chamber to the torque transfer chamber is opened and closed by a valve member provided inside the oil basin chamber which is actuated by a temperature sensing member (provided in front of the closed housing) for sensing the external or or ambient temperature of the closed housing. Therefore, the effective contact area of oil in a torque transfer gap in which a driving disc is disposed is regulated to control the transfer of turning torque to the closed housing (on the driven side).
In the foregoing type of fan coupling device, when the engine comes to a stop with the flow adjusting hole opened due to a high external/ambient temperature, or when the flow adjusting hole is sunk in the oil inside the oil basin chamber during engine stoppage, the amount of oil inside the torque transfer chamber increases due to the natural flow of oil through the flow adjusting hole to the torque transfer chamber.
Accordingly, the conventional pumping mechanism causes "accompanying rotation" on the fan side immediately after engine starting, as illustrated by the characteristic curve B of FIG. 7. This imposes a bad influence on the engine or over-cools the engine, especially in the winter or cold season, to hinder warming up and result in large fan noise.